1. Field of the Invention
The present invention relates to industrial processes emitting fumes which must be cleaned before being discharged to the atmosphere and residues which must most often be treated before utilization or dumping. The present invention relates more particularly to processes employing combustion and of which the residues, including those coming from the cleaning of the combustion fumes, are treated thermically.
2. State of the Art
An industrial process employing combustion, for example combustion of solid and/or liquid fuels, generates fumes in which the impurities of the fuel are taken to high temperature. Such impurities and a part, generally small, of the combustible matter are then either volatilized and entrained with the fumes, or entrained with the fumes in the form of fine large-sized particles resulting most often from a partial fusion followed by an agglomeration and generally called slag. The volatilization of such impurities is all the greater as the temperature is higher, while the quantity of volatilized combustible matter, called unburnt residues, is all the lower as the quality of combustion is better. During cooling of the form are recombined and/or condensed either in the form of new fine particles or on the surface of the fine particles in suspension in the fumes, or, finally, in the form of deposits on the cooling surfaces of a boiler, such deposits most often being re-entrained in the form of particles in the fumes during soot-blowing. A similar phenomenon of recombination/condensation also occurs during the formation and cooling of the slags. After cooling, the fumes laden with dust and gaseous pollutants are cleaned and the resulting residues are either mixed or separated into fly ash, filter cake, etc . . . . For these residues to be of sufficient quality with a view to utilization or dumping thereof, a fuel may be used having very few impurities. However, this type of fuel is expensive and is not always available. There is an increasingly greater advantage in terms of cost in using a fuel containing impurities. And in the case of incineration of waste, there is added to the function of production/energy recovery the function of treatment of waste with a view to utilize the matter or to immobilize the pollutans such as heavy metal. This is why numerous methods are proposed for treating these residues.
For the slags, the virtual vitrification may be improved by optimalizing the thermic trajectory of the slags, for example with an additional burner and slow tempering devices. However; such devices are expensive and complex, which limits use thereof. The slags are ground/screened, after removal of iron in the case of incinerating household refuse, in order to produce, on the one hand, a material utilizable as highway substructure or in building and, on the other hand, fines which must generally be treated like fly ash, as they concentrate the defects in quality of the slags, and may be of comparable granulometry. For fly ash, it has been proposed to inject it, possibly after an appropriate treatment, in the combustion process so as to melt it in the slags. This method is obviously interesting in order to reduce the unburnt residues, but it automatically leads to a considerable enrichment in elements volatilizable in the combustion process. In that case, it is necessary to maintain an outlet for fly ash, much more cancentrated in volatilizable elements and which must be treated. It has also been proposed to treat this fly ash before injection. For example in French Patent Application FR-A-2 547 210 (KERNFORSCHUNGSZENTRUM KARLSRUHE GmbH), the fly ash is leanched with the acid bleed of the wet scrubber placed immediately downstream of an electrofilter for cleaning the fumes. In Patent EP-B-0381601 (LAB S.A.) a forced leaching is effected in a device integrated in one of the hydraulic circuits of the wet scrubber ensuring collection and neutralization of the acid gases of the fumes. In both cases, the greater part of the soluble fraction may be separated and the ashes thus treated no longer cause as great a volatilization when they are injected in the process of combustion. However, the criteria of temperature and of residence time that a correct fusion of the ashes requires can often not be satisfied at the same time as the criteria required by a good combustion, as well as reasonable cost without excessive technical complexity. Moreover, the quality of the slags is limited by the necessity to volatilize little the volatilizable elements which were not eliminated during the treatment before injection, otherwise it is necessary to maintain a fly ash outlet.
It has also been proposed to effect heat treatment of the fly ash in a furnace separate from the combustion process. This method makes it possible to attain the optimal conditions for the heat treatment of the ashes without taking into account the criteria peculiar to the process of combustion. It also allows a better destruction of the residual organic compounds. And the fumes issuing from this furnace are either cleaned separately, which leads to other residues to be treated, or returned to the head of the fume cleaning of which the enrichment in volatilizable elements must be limited by a fly ash outlet.
Finally, it has been proposed to effect the heat treatment by volatilizing the heavy metals as much as possible for the residue thus treated to have a very low content of such elements. The drawback of this method is, on the one hand, of not being able to extract all the heavy metals sufficiently and, on the other hand, of generating a residue comprising heavy metals but also many other elements volatilized during the heat treatment aiming at extracting the heavy metals.